While this is true, it misses the larger issue. There are a number of factors that change driving performance. Eating while driving has a similar effect to talking on a cell phone, much like applying makeup, shaving, etc. In contrast, having a second person (or more) seems to improve driving performance. A young child who is sleeping in the back seat can greatly improve performance, while a small child who is screaming and crying has a negative effect.
If we want to use the logic "it hurts performance", we should ban all electronic devices, and have the radios in cars have 1 big button that can turn on or off road information messages. We should ban all eating while driving. We should require any children that are in cars to be drugged and remain unconscious, and it should be illegal to drive without a second person in the car. That would, statistically, increase driving performance across the board.
Banning cell phones simply because they can be shown to have a negative performance is singling out a single cause because we don't like it. Instead, as usual, the problem is larger and more problematic to fix. Personally, it comes down to how much risk we, as a society, are willing to accept. The current rates of traffic accidents and fatalities are lower than they have been in previous years[*]. Personally, I am comfortable with the current level of risk when I step into a vehicle, either to drive or to ride as a passenger. If society, in general, would like to make changes to improve those numbers, we should have a realistic discussion about what would changes would help, how much each change would help, and what cost would be associated with each change. Simply pointing out one cause, and removing it, without addressing any other issues, is simply punishing a behavior based on personal bias.